Amplifiers are typically used to amplify signals that are output to audio speakers, such as headphones, loudspeakers and/or other audio devices. In wired or non-portable applications, linear amplifiers such as Class A, Class B, and Class AB amplifiers have typically been used. Linear amplifiers include a linear output stage that draws a relatively high bias current while sourcing and sinking current into a load. Therefore, these linear amplifiers consume a relatively high amount of power. Because consumers buying portable audio equipment want to have longer battery life, linear amplifiers are not suitable for use in portable audio applications.
Class D amplifiers have a nonlinear output stage that does not require the high bias current that is used in the linear amplifiers. The increase in efficiency of the output stage, however, is gained at the cost of increased noise and/or distortion. The tradeoff between power consumption and distortion and/or noise has generally been found to be acceptable in portable audio equipment applications.
Referring now to FIGS. 1 and 2, an exemplary Class D amplifier 10 is shown to include a sawtooth waveform generator 14. As can be seen in FIG. 2, a sawtooth signal Vsaw includes a positive sloped portion that increases from a minimum value to a maximum value followed by a return to the minimum value with an almost-infinite negative slope. The sawtooth signal Vsaw is input to an inverting input of a comparator 18. An input signal VIN such as an audio signal is input to a non-inverting input of the comparator 18.
An output of the comparator 18 is input to first and second transistors 20 and 22 that are operated as switches. In this example, the first transistor 20 is a PMOS transistor and the second transistor 22 is an NMOS transistor. The output of the comparator 18 is also inverted by an inverter 24 and input to third and fourth transistors 26 and 28 that are also operated as switches. In this example, the third transistor 26 is a PMOS transistor and the fourth transistor 28 is an NMOS transistor.
Referring now to FIG. 2, the sawtooth signal Vsaw is compared to the input signal VIN. When the input signal VIN is greater than the sawtooth signal Vsaw, the output is high. When the input signal VIN is less than the sawtooth signal Vsaw, the output is low. Alternately, when the input signal VIN is greater than the sawtooth signal Vsaw, the output is low. When the input signal VIN is less than the sawtooth signal Vsaw, the output is high. The transistors 20, 22, 26 and 28 are switched on and off to drive current through a load 40 as depicted in FIG. 1.